They are lashing out against those they deem are their enemy. It's an emotional response from them because in some part of their consciousness, they still have ties to Islam. It's like how some closeted gay dudes hate everything gay to a level that is just beyond normal reaction. They hate themselves for what they are if that makes sense.
So an ex-muslim still has all those emotions that ties them to Islam because that is what they were born with and they hate that part of their identity is still defined by something they think they rejected.
Meaning, it's an emotional response that they have to figure out. You don't get mad at an animal for doing something stupid, pity them and move on.